Chemical Bonding - Intermolecular Forces

Substances that contains covalent bond between their constituent atoms have two possible structures: Either (1) Simple discrete molecules or (2) Macromolecular structure (as known as Giant Molecular Structure). Substances that exist as simple discrete molecules have covalent bond between the atoms, while they have intermolecular forces between the molecules. This is a fact which you must be able to distinguish.

A simple guide I have often used to determine whether a substance exists as either (1) simple discrete molecule or (2) macromolecular, would be to see if I am able to draw the dot and cross diagram of the substance. If I can draw a finite dot and cross diagram showing sharing of electrons, that substance will exist as a simple discrete molecule. The following illustration show examples of finite dot and cross diagrams and their corresponding Lewis structure.

When we can establish that a substance exist as a simple discrete molecule, we will need to determine the shape of its molecule. The shape of the molecule play a role in determining the intermolecular forces that exist between the molecules. The pictures below provide a guide to how to determine the shape of the molecule.

There are two broad types of intermolecular forces, van der Waals Forces and Hydrogen-bonding. The former can be divided to (1) Induced dipole - induced dipole (id-id) interactions and (2) Permanent dipole - permanent dipole (pd-pd) interactions.

All molecules contain id-id interaction as this form of interaction is due to a temporal unequal distribution of the molecule's electron cloud, which result in a side of the molecule to have smaller electron density, while the other side has more.

Non-polar molecules have only id-id interaction between their respective molecules.

While polar molecule can have either pd-pd interactions or Hydrogen-bonding as the pre-dominant (note the I used pre-dominant instead of "only") intermolecular forces.

Hydrogen-bonding is a unique case of pd-pd (thus, Hydrogen-bonding is an intermolecular force of attraction). Polar molecules which has a highly electronegative atom such as N, F, and/or O and this electronegative atom is attached to O will show Hydrogen-bonding.

With the knowledge of shapes of molecules and intermolecular forces, we will soon be able to effectively discuss how to account certain physical properties of the substances.

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Article written by Kwok YL 2009.}

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